1. Field of the Invention
The subject invention relates to a burn-in socket for a flatpack semiconductor package.
2. Description of the Prior Art
Semiconductor packages are arranged with several lead configurations denoting their use. One semiconductor package is known as the flatpack package and includes a plurality of leads extending from the package body with all leads arranged in a common flat plane. Typically all packages are tested in some manner to ensure their proper functioning, including burn-in testing where the devices are inserted into sockets and installed within large convection ovens and the packages are operated at elevated temperatures.
The package known as the flatpack semiconductor is a very fragile component, and as such, the package is typically installed within a carrier which includes an insulative housing to surround the package body, and a plurality of channels in which the leads reside. The carrier is placed within the socket during the burn-in testing and, when completed, the carrier and package are removed and the semiconductor package is shipped within the carrier for protection of the package leads. When the packages arrive at the manufacturer of the computer, or other user, the leads are partially cut off and the leads are prepared for surface mounting onto printed circuit boards.
The size and configuration of the burn-in sockets are dictated partially by the package geometry and partially by the burn-in facility, as sockets have to be compatible with the preexisting footprint of the burn-in printed circuit boards. The burn-in boards are printed circuit boards which are densely filled with sockets to receive the packages and carriers. The burn-in boards have traces to the sockets which power up the packages during their burn-in testing.
As it is a requirement in burn-in applications to maintain power to the package leads throughout the burn-in testing, it is a requirement specified by most package manufactures to maintain a constant contact force on the package leads in the range of 80-100 grams when the package is inserted within the sockets. Any contact force lower than 80 grams can result in discontinuity between the socket contacts and the package leads resulting in a loss of power to the package leads. A loss of power to the package, for any time frame during the burn-in cycle, would result in a scrapped package, as the packages are rarely tested twice as the heat effect alone on the package could be detrimental to its life. Thus if a package is not properly connected during the burn-in test, the package is discarded rather than retested.
One difficulty which has been experienced with present burn-in sockets in that the socket contacts are characterized by a steep force deflection curve, that is, a small change in deflection of the socket contacts results in a large increase in the contact force on the package leads. This is a disadvantage in the burn-in application as the conventional carriers are supplied by a variety of manufacturing sources, and the carriers can vary in thickness by 0.006 inches from each other. This differential in thickness directly results in an equal insertion depth differential, that is, the differential thickness is directly additive to the overall insertion depth. Given the step force deflection curve of up to 10 grams/0.001 inch, the contact force on several of the available sockets can vary drastically, as low as 40 grams on the lower end of the deflection while yielding the socket contact at the upper end of the deflection. The yielding of the contacts is due to the large concentration of bending stress located at the bending radius.
Another difficulty which has been experienced with present burn-in sockets relates to the fragility of the package leads. Several package manufacturers have leads which are made of KOVAR (KOVAR is the trademark of Carpenter Technology Corporation for an iron-nickel-cobalt alloy) overplated with gold. The difficulty relates to the configuration of the socket contacts themselves, the contacts having a tuning fork shape with one of the forks lying in the horizontal plane adjacent to the housing floor, while the other end of the fork includes an upwardly extending projection which forms the contact point which has a very small radius. If the contact force on the contact is high, the small radiused contact can mark or dimple the package lead which results in a defective package, causing the manufacturer to discard the package.
Another difficulty which has arisen with the burning in of semiconductor components relates to the buildup of oxides on the contact lead which results in a lack of contact between the socket contact and the package lead. As several of the available socket contacts are configured as the tuning fork arrangement described above, the contact's movement during the insertion of the package is only vertical, which provides no wiping action between the socket contact and the package lead. Wiping action typically clears the oxides from the package leads and provides a clean surface area for the contact to mate with resulting in a good electrical connection between the socket contacts and leads.
Other difficulties which have arisen relate to the fact that the packages are presently hand installed and removed from the sockets. Most sockets include an opening beneath the package body which allows heat dissipation from the package body during the test. When the packages are finished with the burn-in cycle, a bladed tool is inserted underneath the package and pulled upwardly to remove the package and carrier. It is common for the person removing the package to snag the end of the contacts with the end of the bladed tool which breaks or damages the socket contacts. As numerous sockets are installed on the burn-in boards, the damaging of the sockets results in a loss in burn-in production; either time is wasted by replacing the damaged socket, or if the socket is not replaced, the damaged socket is incapable for use thereby wasting valuable real estate on the burn-in board.
It is another disadvantage of the present burn-in sockets that the latching arrangements are such that they rotate about one end only, covering the top of the socket and carrier and latching to the housing at the end opposite its rotation point. This makes it difficult for assembly equipment to install and remove the carriers as the latch must be held and moved in simultaneous vertical and translational motion. Although it is possible to load and unload these sockets with an assembly machine having a multi-directional hand, such as a three directional robotic assembly machine, it would be virtually impossible for use with assembly equipment having only vertical, or one directional movement. It can be appreciated that it is highly desirable to have the ability of loading and unloading the carriers and packages with assembly equipment having a single direction of travel, as the cost of such is comparably low as compared to sophisticated robotics having multi-directional movement capabilities.